This invention relates to polymeric dielectrics and in particular to techniques for improving the electrical breakdown strength of such dielectrics.
Polymeric materials are widely used as insulators under high voltage stress conditions. Typical applications include power cables and high voltage bushings. In the design of such structures due allowance must be made for the electrical breakdown strength of the polymeric material.
The bulk electrical breakdown field strength of a polymeric material depends both on the nature of the material and, in many cases, on the thermal and mechanical treatment of the material prior to use. For example, where the polymer is to be applied to a cable by extrusion, the mechanical and thermal stress involved can have a significant effect on spherulite size.
The number of factors can affect the breakdown field strength of a material. For thermoplastics such as polyethylene and polypropylene an important factor is the crystal structure of the material. Such materials comprise spherulites which are substantially spherical radiating arrays of crystallites. These spherulites are typically of 40 to 60 microns in diameter and compose the entire structure of the polymer. They may be thought of as analogous to grains in metal with spherulite size being analogous to grain size. The way in which these spherulites affect the breakdown strength is not fully understood, but it has become apparent that a large average spherulite diameter has an adverse effect on the breakdown field strength.